Estimating mud expulsion rates from temperature measurements on Håkon Mosby Mud Volcano, SW Barents Sea

Marine Geology 229 (2006) 1 – 14 www.elsevier.com/locate/margeo

Estimating mud expulsion rates from temperature measurements on Håkon Mosby Mud Volcano, SW Barents Sea N. Kaul a,⁎, J.-P. Foucher b , M. Heesemann a a

Universität Bremen, Geowissenschaften, Klagenfurter Strasse, 28359 Bremen, Germany b IFREMER, BP70, 29280 Plouzane, France

Received 10 May 2005; received in revised form 2 February 2006; accepted 9 February 2006

Abstract The Håkon Mosby Mud Volcano (HMMV), located on the Norwegian–Barents–Svalbard continental margin in 1250 m water depth, has been identified and described as a structure caused by upward transport of mud, pore water and gas [e.g. Eldholm, O., Sundvor, E., Vogt, P.R., Hjelstuen, B.O., Crane, K., Nilsen, A.K., Gladczenko, T.P., 1999. SW Barents Sea continental margin heat flow and Håkon Mosby Volcano. Geo-Marine Letters 19, 29–37]. During RV Polarstern expedition ARK XIX/3b in 2003, an integrated study took place to investigate the detailed morphology, biology, chemistry and geophysical aspects of HMMV [Klages, M., Thiede, J., Foucher, J.-P., 2004. The Expeditions ARK XIX 3a, 3b and 3c, Berichte zur Polarforschung, 488.]. In this paper, we describe a detailed survey involving more than 100 temperature gradient measurements in order to reveal the temperature structure of HMMV. Values of apparent heat flow up to 3000 mW/m2 and absolute temperatures up to 25.8 °C have been detected. These enormously high heat flux values do not reflect deep-seated thermal gradients but are a result of surface mud flows. The occurrence and abundance of mud flows varies in different areas within the HMMV inner crater. A combination of very shallow, ROV-derived temperature measurements, a large number of intermediate depth temperature gradients and nine deep penetrating gravity corers allows us to reconstruct the history of activity of HMMV. Modeling of the observed temperature gradients indicates vigorous activity with very recent mud flows. Furthermore, the supply of energy and material from the HMMV is estimated to have an annual mass volume of ∼15,000 m3. From the thermal corona of HMMV and the observed horizontal heat flux, it is possible to deduce that the long term temperature of the mud pond is stable at a temperature approximately similar to the recent maximum temperature of 25 °C. © 2006 Elsevier B.V. All rights reserved. Keywords: seafloor hydrothermal system; mud volcano; heat flow; convection; SW Barents Sea

1. Introduction

⁎ Corresponding author. Universität Bremen, FB5, PO box 330440, 28334 Bremen, Germany. Tel.: +49 421 218 7162; fax: +49 421 218 7163. E-mail addresses: [email protected] (N. Kaul), [email protected] (J.-P. Foucher), [email protected] (M. Heesemann). 0025-3227/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2006.02.004

During the multi-discipline R/V Polarstern cruise ARK XIX/3b (27 June 2003 to 19 July 2003), investigations of Håkon Mosby Mud Volcano (HMMV) were carried out to better understand the thermal structure of this 1 km wide pond. A review of heat flow measurements and the thermal field in the Norwegian–Greenland Sea associated with HMMV

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was given by Eldholm et al. (1999), while the thermal evolution of the Svalbard Margin was investigated systematically by Crane et al. (1988) (Fig. 1). These studies reveal a regional background heat flow of 53– 75 mW/m2 with only slight excursions up to 122 mW/m2 observed when crossing the Senja Fracture Zone (SFZ) (Sundvor et al., 2000). Heat flow values decrease slightly from the Lofoten Basin in the West (water depth of 2800 m) to the SW Barents Sea in the East (water depth of only 500 m). Interpretation of seismic data (Hjelstuen et al., 1999) shows that as water depth decreases, the section of pre-glacial and glacial sediments on top of oceanic crust increases from

∼3 km to a maximum of ∼6 km. The HMMV is situated near the maximum sediment thickness within the basin and is underlain by 3 km of glacial sediment plus 3.2 km of pre-glacial sequences. Ties to commercial wells indicate an age of 2.3 Ma for the base of the glacial unit (Fiedler and Faleide, 1996), implying an extremely high sedimentation rate of 135 cm/1000 yr for the last 2.3 Ma. Based on magnetic anomalies, an age of 33–37 Ma is estimated for the underlying oceanic crust (Hjelstuen et al., 1999). Calculating a background heat flow according to the general model of Parsons and Sclater (1977) predicts a value of ∼80 mW/m2.

Fig. 1. Working area in the southwest Barents Sea and regional heat flow determinations. Håkon Mosby Mud Volcano (HMMV) is situated at a scarp on the flank of the Barents–Svalbard continental margin. Bathymetry data from ETOPO2 (NGDC, 2004) dataset allows to identify this depression. Age of oceanic crust is taken from Müller (1997). HMMV area is closely sampled with temperature measurements.

N. Kaul et al. / Marine Geology 229 (2006) 1–14

Eldholm (1999) reported unusually high heat flow values, up to 1000 mW/m2 at HMMV that indicate convective heat transport. At the same time, optical imagery observed features attributed to gas or fluid expulsion (Vogt et al., 1997). Therefore, detailed thermal investigations were planned along with numerous geological work on methane gas and gas hydrate occurrences at HMMV. This structure has been inferred as an active mud volcano due to outflow channels, observed by side scan sonar observations (Vogt et al., 1999). Taking the terminology for mud structures (i.e. Camerlenghi et al., 1992; Cita et al., 1994; Camerlenghi et al., 1995; Kopf, 2002), the structure is a mud pie (flank slope < 5°) rather than a volcano, signifying production of low viscosity mud. Gas hydrates have been recovered during coring (Ginsburg et al., 1999). As a consequence, we expect a thermal field, which is heavily affected by transient effects and mass convection. Under these circumstances, it would be misleading to use the term heat flow determinations, as this requires at least a quasi-steady-state situation for a reasonable long time period. We rather refer to temperature measurements and processes they describe.

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3.5 m penetration with 11 sensors distributed over a length of 3 m at a spacing of 0.3 m. Furthermore, this probe determines in-situ thermal conductivity at all 11 thermistor positions (Hyndman et al., 1979). To extend “shallow” temperature measurements to greater depth, a gravity corer was equipped with autonomous temperature probes. Ten instruments of Micrel THP type (for specifications see above) were welded onto the core barrel at 0.88 or 1 m spacing. Six gravity corer stations gave reliable temperature measurements up to a maximum depth of 16 m. Three stations failed due to bent core barrels. We attribute this to rigid layers of gas hydrate located several meters below the seafloor, which could not be penetrated. 3. Thermal measurements A total number of 98 successful temperature gradient determinations were achieved with the Bremen heat

2. Instrumentation Three different tools were used to measure a large number of temperature gradients in the shallow (< 1 m), intermediate (0–4 m) and “deep” (up to 16 m below seafloor (mbsf)) depth range. The lateral resolution varied from 50 m to some hundred meters, depending on the instrument used. During this cruise, it was possible to take advantage of remotely operated vehicle (ROV) “Victor 6000” for shallow and closely spaced temperature measurements. During several dives, shallow seafloor temperature gradients at a spacing of 50 m were obtained. A temperature lance fitted with two Micrel THP type autonomous data loggers was used (Géli et al., 2001). These temperature loggers have a resolution of better than 1 mK at 20 °C and a capacity of ∼25000 samples with data compression. The housing has a diameter of 28 mm and is rated at 600 bar. Two of these sensors were mounted 0.25 and 0.55 m below the top of a lance, which could be handled by the ROVs manipulator arm (Fig. 2). Penetration depth was controlled very accurately by “Victor 6000” so that we were confident to get an accurate temperature gradient for the uppermost decimeters below the mud line. For the medium depth range between 0.8 and 4 m, a conventional heat flow probe for multipenetration stations was used. The Bremen heat probe is a violin bow instrument of Lister-type design and capable of

Fig. 2. Mini-temperature lance equipped with two MICREL THP temperature sensors.

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Fig. 3. High resolution bathymetry of Håkon Mosby Mud Volcano, produced by IFREMER, using deep towed multi-beam echo sounders, mounted beneath ROV “Victor 6000”. Overlain are positions of heat flow sites (white dots) and gravity corers (blue stars). A regional classification is (i) outside the moat, (ii) the moat ring, (iii) a rough area north and west of the centre and (iv) a smooth area expanding from the centre southwards. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

probe and an additional nine vertical profiles were made with a gravity corer, equipped with thermistors, plus two horizontal profiles of shallow temperature measurements with the mini-lance, operated with “Victor 6000”. All measurements are concentrated on the HMMV crater, including its moat. The survey across HMMV was planned for detailed investigation, so that distances between points in a given line are as short as 100 m across the center, and 200–300 m outside that region (Fig. 3). From the very first profile (H0310), we learned that temperature anomalies are very distinct and limited in extent. Therefore, survey lines were planned to make a grid with many crossings near the center of HMMV. Eldholm et al. (1999) showed that background heat flow values of 55–70 mW/m2 are present at a distance of only a few kilometers from HMMV. Thus, we did not extend our profiles more than 10 km from the center. The seafloor conditions are very favorable for instrument penetration down to 4 mbsf. Fig. 3 shows the distribution of these measurements overlain on

high resolution bathymetry. The bathymetric map is a result of a systematic survey by the deep flying ROV “Victor 6000”. Detailed surveys with the mini-lance were located at key sites, chosen from previous heat flow investigations or visual inspection by ROV “Victor 6000”. Navigation of the ROV is relative to the surface vessel by means of a Posidonia™ acoustic transponder. True ground positions were accurate within ± 30 m, deduced from the relocation of known features. Only few sites were positioned outside the crater, up to 10 km away, in order to acquire a regional reference for temperatures and gradients. Due to a very soft seafloor consistency in the center of HMMV, we have to consider “overpenetration” for the heavy instruments, i.e. penetration is larger than the maximum length of the lance and thus poorly constrained. The total penetration has then to be estimated by the mud line on the deep sea cable and the extrapolation of the thermal gradient (Table 1).

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Table 1 Interpolated temperatures at relative depths with respect to the uppermost sensor Longitude 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57

14.7064 14.7115 14.7167 14.7201 14.7235 14.7252 14.727 14.7287 14.7304 14.7321 14.7355 14.7406 14.7458 14.7492 14.7528 14.7615 14.7112 14.7141 14.717 14.7199 14.7228 14.7257 14.7288 14.7316 14.7345 14.7374 14.7403 14.7432 14.7461 14.7287 14.7287 14.7287 14.7287 14.7287 14.7287 14.7287 14.7287 14.7287 14.7287 14.7217 14.7216 14.7215 14.7213 14.7212 14.7211 14.721 14.721 14.7209 14.7208 14.7206 14.7205 14.7204 14.7203 14.7202 14.7201 14.72 14.7199

Latitude 72.0136 72.0114 72.0092 72.0078 72.0063 72.0056 72.0049 72.0042 72.0034 72.0027 72.0012 71.9991 71.9969 71.9954 71.9933 71.9893 72.0042 72.0042 72.0042 72.0042 72.0042 72.0042 72.0042 72.0042 72.0042 72.0042 72.0042 72.0042 72.0042 72.0087 72.0078 72.0069 72.006 72.0051 72.0043 72.0033 72.0024 72.0015 72.0006 72.0255 72.0237 72.0219 72.0201 72.0183 72.0165 72.0147 72.0129 72.0111 72.0093 72.0067 72.0057 72.0033 72.0021 72.0003 71.9985 71.9967 71.9949

HF-name

k

T_0m

T_1m

T_2m

T_3m

Gradient (mK/m)

PS64/station

H0310P01 H0310P02 H0310P03 H0310P04 H0310P05 H0310P06 H0310P07 H0310P08 H0310P09 H0310P10 H0310P11 H0310P12 H0310P13 H0310P14 H0310P15 H0310P16 H0311P01 H0311P02 H0311P03 H0311P04 H0311P05 H0311P06 H0311P07 H0311P08 H0311P09 H0311P10 H0311P11 H0311P12 H0311P13 H0312P01 H0312P02 H0312P03 H0312P04 H0312P05 H0312P06 H0312P07 H0312P08 H0312P09 H0312P10 H0313P01 H0313P02 H0313P03 H0313P04 H0313P05 H0313P06 H0313P07 H0313P08 H0313P09 H0313P10 H0313P11 H0313P12 H0313P13 H0313P14 H0313P15 H0313P16 H0313P17 H0313P18

1.11⁎ 1.11 1.11 1.40⁎ 1.08⁎ 1.08 1.08 1.02⁎ 1.00⁎ 0.96⁎ 1.27⁎ 1.19⁎ 1.13⁎ 1.11⁎ 1.12⁎ 1.12⁎ 1.00 1.03⁎ 1.12⁎ 1.18⁎ 1.00 1.00 1.00 0.69⁎ 1.00 1.15⁎ 1.00 1.34⁎ 1.16⁎ 1.16⁎ 1.11⁎ 1.22⁎ 1.7⁎ 0.73⁎ 1.19⁎ 1.10 0.99⁎ 1.13⁎ 1.11⁎ 1.1 1.1 1.09⁎ 1.1 1.09⁎ 1.1 1.13⁎ 1.15⁎ 1.1 1.1 1.10⁎ 1.06⁎ 1.1 1.1 1.22⁎ 1.30⁎ 1.32⁎ 1.1

− 0.76 − 0.75 − 0.69 − 0.48 − 0.5 5 24.5 11.5 4.5 7 − 0.68 − 0.78 − 0.8 − 0.8 − 0.8 − 0.81 − 0.8 − 0.51 0.5 1.2 23 16 13.6 10.1 8.9 − 0.2 − 0.6 − 0.72 − 0.75 − 0.5 − 0.2 − 0.8 − 0.8 4.5 2.5 10 4 5 − 0.5 − 0.79 − 0.78 − 0.78 − 0.77 − 0.78 − 0.77 − 0.75 − 0.72 − 0.7 − 0.65 − 0.3 − 0.8 17 7 − 0.6 − 0.73 − 0.75 − 0.76

−0.7 −0.69 −0.6 −0.31 2 5.5 26.0 12 6.5 9 −0.59 −0.71 −0.74 −0.74 −0.74 −0.75 −0.58 −0.1 1.4 2.8 23.8 18 13.8 11.8 10 0.3 −0.45 −0.64 −0.68 −0.3 0.5 −0.2 −0.6 6.2 4.4 12 6 7 −0.2 −0.73 −0.73 −0.73 −0.72 −0.72 −0.72 −0.69 −0.65 −0.63 −0.57 0.12 0 19.5 10 −0.2 −0.66 −0.69 −0.71

− 0.63 − 0.52 − 0.52 − 0.05 3.5 8 25.4 14.5 8.4 11 − 0.5 − 0.64 − 0.67 − 0.68 − 0.68 − 0.69 − 0.42 0.3 2.3 4.5 24.2 20 14.1 13.4 11 0.9 − 0.3 − 0.56 − 0.6 − 0.06 1.2 0.3 0.2 8.3 6.2 14.2 8 8.6 0.1 − 0.68 − 0.67 − 0.66 − 0.66 − 0.66 − 0.65 − 0.62 − 0.59 − 0.56 − 0.48 0.52 2 22.3 13.5 0.3 − 0.60 − 0.63 − 0.65

−0.56 −0.56 −0.41 0.15 5.0 10.5 25.2 17 10.5 13.3 −0.41 −0.62 −0.61 −0.62 −0.62 −0.64 −0.27 0.8 3.1 6.2 24.6 22.5 14.4 15 12 1.5 −0.17 −0.5 −0.56 0.15 1.9 1.0 0.9 10.1 8 16.5 10 10.8 0.39 −0.62 −0.62 −0.61 −0.6 −0.6 −0.59 −0.55 −0.51 −0.49 −0.39 0.9 4 25 17 0.8 −0.52 −0.56 −0.58

67 66 86 240 2740 2430 − 98 2500 2050 2010 90 70 62 61 60 57 153 433 936 1760 402 2000 325 1590 1070 564 138 72 75 229 708 ? 834 1990 1870 2140 2090 1910 275 53 55 58 59 59 61 66 71 73 89 418 2090 2920 2820 479 69 59 60

PS64/319-1 PS64/319-2 PS64/319-3 PS64/319-4 PS64/319-5 PS64/319-6 PS64/319-7 PS64/319-8 PS64/319-9 PS64/319-10 PS64/319-11 PS64/319-12 PS64/319-13 PS64/319-14 PS64/319-15 PS64/319-16 PS64/333-1 PS64/333-2 PS64/333-3 PS64/333-4 PS64/333-5 PS64/333-6 PS64/333-7 PS64/333-8 PS64/333-9 PS64/333-10 PS64/333-11 PS64/333-12 PS64/333-13 PS64/333-14 PS64/333-15 PS64/333-16 PS64/333-17 PS64/333-18 PS64/333-19 PS64/333-20 PS64/333-21 PS64/333-22 PS64/333-23 PS64/343-1 PS64/343-2 PS64/343-3 PS64/343-4 PS64/343-5 PS64/343-6 PS64/343-7 PS64/343-8 PS64/343-9 PS64/343-10 PS64/343-11 PS64/343-12 PS64/343-13 PS64/343-14 PS64/343-15 PS64/343-16 PS64/343-17 PS64/343-18 (continued on next page)

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Table 1 (continued)

58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98

Longitude

Latitude

HF-name

k

T_0m

T_1m

T_2m

T_3m

Gradient (mK/m)

PS64/station

14.7198 14.7142 14.7088 14.717 14.7197 14.7224 14.7252 14.7279 14.7308 14.7337 14.7366 14.7396 14.7425 14.7454 14.7512 14.7348 14.7348 14.7348 14.7348 14.7348 14.7348 14.7348 14.7348 14.7348 14.7359 14.7348 14.7348 14.7317 14.7479 14.7642 14.7804 14.7966 14.8129 14.8291 14.8453 13.4328 13.4333 13.4340 13.4353 13.4527 13.4489

71.9931 72.0013 72.0007 72.0016 72.0019 72.0022 72.0025 72.0028 72.0028 72.0028 72.0028 72.0028 72.0028 72.0028 72.0028 71.9964 71.9982 72 72.0009 72.0018 72.0027 72.0036 72.0045 72.0054 72.0059 72.0072 72.009 72.0003 71.9929 71.9854 71.978 71.9705 71.963 71.9556 71.9481 71.7018 71.7000 71.6982 71.6946 71.6589 71.6571

H0313P19 H0314P01 H0314P02 H0314P03 H0314P04 H0314P05 H0314P06 H0314P07 H0314P08 H0314P09 H0314P10 H0314P11 H0314P12 H0314P13 H0314P14 H0315P01 H0315P02 H0315P03 H0315P04 H0315P05 H0315P06 H0315P07 H0315P08 H0315P09 H0315P10 H0315P11 H0315P12 H0316P01 H0316P02 H0316P03 H0316P04 H0316P05 H0316P06 H0316P07 H0316P08 H0317P01 H0317P02 H0317P03 H0317P04 H0317P05 H0317P06

1.1 1.34⁎ 1.1 1.22⁎ 1.1 1.21⁎ 0.95⁎ 1.17⁎ 1.30⁎ 1.09⁎ 1.13⁎ 1.23⁎ 1.30⁎ 1.22⁎ 1.10⁎ 1.16⁎ 1.25⁎ 1.34⁎ 1.15⁎ 1.27⁎ 1.23⁎ 1.20⁎ 1.14⁎ 1.1 na 1.1 1.20⁎ 1.32⁎ 1.12⁎ 1.1 1.36⁎ 1.1 1.20⁎ 1.1⁎ 1.23⁎ 1.21⁎ 1.21⁎ 1.1 1.1 1.14⁎ 1.15⁎

− 0.77 − 0.74 − 0.6 − 0.6 9 − 0.51 13 8 7.9 − 0.2 − 0.7 − 0.6 − 0.7 − 0.72 − 0.76 − 0.74 − 0.74 − 0.71 − 0.71 − 0.6 − 0.6 − 0.3 0.2 0.0 na − 0.2 − 0.76 − 0.61 − 0.79 − 0.8 − 0.81 − 0.8 − 0.79 − 0.77 − 0.79 − 0.79 − 0.78 − 0.78 − 0.8 − 0.83 − 0.81

− 0.71 − 0.66 − 0.5 − 0.45 10.5 − 0.1 14.4 10 9.3 0.4 0.0 − 0.47 − 0.63 − 0.65 − 0.7 − 0.67 − 0.66 − 0.65 − 0.64 − 0.49 − 0.2 0.6 1.0 0.6 na 0.4 − 0.6 − 0.4 − 0.74 − 0.75 − 0.77 − 0.75 − 0.74 − 0.72 − 0.75 − 0.73 − 0.73 − 0.73 − 0.75 − 0.77 − 0.77

− 0.65 − 0.6 − 0.41 − 0.1 11.8 0.56 15.8 11 10.9 1.1 0.7 − 0.33 − 0.56 − 0.57 − 0.63 − 0.61 − 0.60 − 0.59 − 0.56 − 0.36 0.3 0.9 1.8 1.4 na 1.0 − 0.4 − 0.2 − 0.68 − 0.69 − 0.72 − 0.7 − 0.7 − 0.67 − 0.7 − 0.68 − 0.68 − 0.68 − 0.7 − 0.72 − 0.72

− 0.59 − 0.54 − 0.31 0.25 13 na 17.1 11.5 13.3 1.55 na − 0.2 − 0.5 − 0.51 − 0.56 − 0.55 − 0.54 − 0.53 − 0.49 − 0.25 0.8 na 2.7 2.2 na 1.6 na 0.0 − 0.62 − 0.64 − 0.68 − 0.65 − 0.66 − 0.63 − 0.65 − 0.64 − 0.65 − 0.65 − 0.65 − 0.66 − 0.66

58 59 93 290 1310 930 1440 1150 1540 688 677 127 68 70 65 65 65 63 78 123 480 918 836 756 na 637 195 213 58 54 42 50 43 46 47 48 46 42 51 52 49

PS64/343-19 PS64/361-1 PS64/361-2 PS64/361-3 PS64/361-4 PS64/361-5 PS64/361-6 PS64/361-7 PS64/361-8 PS64/361-9 PS64/361-10 PS64/361-11 PS64/361-12 PS64/361-13 PS64/361-14 PS64/361-15 PS64/361-16 PS64/361-17 PS64/361-18 PS64/361-19 PS64/361-20 PS64/361-21 PS64/361-22 PS64/361-23 PS64/361-24 PS64/361-25 PS64/361-26 PS64/369-1 PS64/369-2 PS64/369-3 PS64/369-4 PS64/369-5 PS64/369-6 PS64/369-7 PS64/369-8 PS/64/389-1 PS/64/389-1 PS/64/389-1 PS/64/389-1 PS/64/389-2 PS/64/389-2

Conductivity values k⁎ are calculated from measurements, others are spatially interpolated.

3.1. Surface measurements using a mini-lance

3.2. Heat flow probe temperature determinations

A closer look at the data from the mini-lance shows temperatures of 0 °C to +13 °C at 0.25 mbsf and +0.1 °C to +22 °C at 0.55 mbsf (Fig. 4). The maximum resulting temperature gradient within the uppermost half meter of the sediment is as high as 41,000 mK/m, assuming a bottom water temperature of − 0.8 °C. This is far more than can be sustained by basal conductive heat flow, as this would lead to unrealistic high subseafloor temperatures, i.e. 1000 °C in only 25 m depth. This leads to the question: How deep do these high gradients reach?

Usage of the heat flow probe gives information on the temperature distribution down to ∼3.5 mbsf at 92 locations within the crater and the immediate surrounding (up to 2 km distance from the center) and at six locations at a distance of ∼45 km from the center. An example of one profile crossing HMMV from NW to SE is given in Fig. 5. The compilation of all data points in the vicinity of HMMV is given in Fig. 6. The singularity of the temperature distribution becomes evident from the smoothed x–y–t plot. The figure displays the temperature of the uppermost sensor

N. Kaul et al. / Marine Geology 229 (2006) 1–14

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Fig. 4. Temperature measurements as time series of the two MICREL sensors, mounted onto a mini-lance. Sensor positions are 55 (black line) and 25 cm (grey line) below the top of the lance. The position of the maximum temperature coincides with the highest temperatures found with the Bremen heat probe. Numbers denote the vertical temperature gradient in °C/m.

Fig. 5. Temperature–depth plots of station H0310, aligned in a NW–SE direction across the centre of HMMV. Temperature scales of figures e–j are compressed by a factor of 13 to accommodate for the higher temperatures within the central crater. At the very centre (H0310P07, g), high temperatures correlate with a low gradient.

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Fig. 6. Compilation of heat flow probe temperature data of the uppermost sensor (app. 0.7–0,8 mbsf) of all locations near HMMV. The upper part shows a smoothed version of the dataset, while the lower part displays the irregular distributed data points and their projections onto the base lines. Concentric circles indicate the position of inner and outer rim of HMMV.

(∼0.8 mbsf) which shows the most pronounced special variation. At a range of 1 km from the rim of the crater, we find seafloor temperatures as low as −0.8 °C immediately beneath the mud line, slightly increasing downward in the sediments, resulting in a vertical gradient of 55–65 mK/m within the uppermost 3 m. Thermal conductivity determinations, derived from in situ measurements, give values of 1.0–1.2 W/m K increasing downwards, with a mean value of ∼1.1 W/m K. This results in heat flow values of 60–73 mW/m2, a heat flow which has been found for the region within several hundred kilometers of the HMMV (Eldholm et al., 1999). Therefore, this station can be considered a reference station which is unaffected by the mud volcano.

A completely different observations is found at a location slightly NW of the geometrical center of the crater where an almost homogeneous temperature distribution at a temperature of + 25 °C exists (Fig. 5g). A closer investigation of this site reveals that the temperature distribution has a clear maximum of 25.8 °C at 1–2 m depth, and decreasing temperatures both upward (24.7 °C) and downward (24.8 °C). This is the highest temperature encountered during this and all other published surveys of HMMV. The majority of measurements exhibit a more or less linear, positive gradient. Values range from ∼60 mK/m to 2700 mK/m next to the center. Strong indications of convective heat transport within the pore space are indicated by curved gradients, observed occasionally. The curvature of

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Fig. 7. Data from six successful gravity corers. (a) GC 2 located near the centre, (b–c) between centre and moat rim and (f) outside the moat rim. Temperature scale is identical −2 to +27 °C.

convex shaped thermal gradients was evaluated at eight locations, implying upward migration velocities of pore water of 0.3–6 m yr− 1. The distribution of fluid convection derived from chemical and thermal gradients is discussed in detail in de Beer et al. (2006). Looking at the distribution of temperatures, we find that a huge horizontal gradient exists. Taking values at 3–4 m depth, we find an overall horizontal gradient of 28.2 °C/ km from a “normal” background gradient to the center (i.e. H0310P02–H0310P07, Fig. 5b–g). Across the moat rim of the crater, we find an enormous temperature difference of 13 °C over a horizontal distance of 200 m, indicating a gradient of 65 °C/km (H0310P10– H0310P11, Fig. 5j and k). Both vertical and horizontal gradients give evidence for a very active system, which is able to maintain such huge temperature gradients.

3.3. Gravity corers with attached temperature probes Long gravity corers equipped with temperature probes allowed determination of deep temperature profiles within and outside the crater. Five sites are located near the center (GC 2, GC 4, GC 6, GC 7 and GC 8) and one site (GC 3) is positioned SE outside the rim (Fig. 3). Fig. 7a–f displays the temperature–depth correlations. Site GC 3 outside the crater shows a normal positive gradient of ∼79 mK/m in good agreement with neighboring 3 m deep heat flow stations. At all other sites, gradients are not at all linear but tend to reach a maximum value in an asymptotic manner. A temperature of 23 °C is never exceeded at the depth range of the gravity corers (see Table 2). However, the asymptotic temperature increase is at its limit in two places (GC 2

Table 2 Temperature data of seven successful gravity corer measurements Name

Latitude

Longitude

GC1 GC2

72°0.30′N 72°0.28′N

14°43.60′E 14°43.57′E

GC3

72°0.02′N

14°44.13′E

GC4

72°0.13′N

14°43.81′E

GC5

72°0.17′N

14°43.85′E

GC6

72°0.20′N

14°43.88′E

GC7

72°0.20′N

14°43.88′E

GC8

72°0.21′N

14°43.66′E

GC9

72°0.38′N

14°43.61′E

No data 0 −0.8 0 −0.8 0 −0.8 No data – 0 −0.8 0 −0.8 0 −0.8 0 −0.8

5.52 18.95 0.94 − 0.78 7.69 15.13

6.46 19.15 1.9 − 0.73 8.65 16.25

7.46 19.22 2.85 − 0.64 9.6 17.3

8.46 19.4 3.9 − 0.56 10.65 17.36

9.45 19.59 5.52 − 0.51 12.27 19.6

6.46 − 0.36 13.21 20.35

7.46 − 0.29 14.21 20

8.46 − 0.24 15.21 21.94

9.45 − 0.19 16.2 22.31

5.89 12.8 5.89 15.8 2.89 11.65 0.72 0.35

6.72 14.82 6.72 18.1 3.72 14.01 1.58 0.67

7.58 16.06 7.58 19.55 4.58 15.31 2.45 1.87

8.45 17.16 8.45 20.61 5.45 16.76 3.31 3.13

9.31 18.68 9.31 21.72 6.31 18.46

10.89 20.11 10.89 22.31 7.89 19.71

11.74 20.72 11.74 22.59 8.74 20.4

12.6 20.21 12.6 22.55 9.6 21.2

13.46 22.43 13.46 22.68 10.46 21.66

14.34 22.74 14.34 22.64 11.34 22.11

[m] [°C] [m] [°C] [m] [°C] [m] [°C] [m] [°C] [m] [°C] [m] [°C] [m] [°C]

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and GC 7), but not at sites GC 4, GC 6 and GC 8, suggesting even higher temperatures further below. It is noteworthy that the highest temperature of almost 26 °C has been found in the uppermost 4 mbsf and not in the deeper section. 4. Calculation of mud and fluid flow From horizontal and vertical temperature distributions, we derived a model of eruptive mud expulsions from a single channel near the geometrical center of HMMV. As a temperature-difference of more than 25 °C to ambient environment is maintained within a considerable area of the crater, we have to suppose a heat supply at a rate, high enough to counteract conductive cooling. This leads to a number of questions: how often do eruptions occur? Can we determine the maximum temperature of a mud flow? Are estimates possible for the history of eruption in certain areas, deduced from temperature measurements? As input dataset for modeling a singular mud flow, a combination of data at the central location is possible because individual positions of measurements are less than 50 m apart. We combine stations H0310P07 and GC 2 with data from the mini-lance in order to yield a 9.5 m long temperature profile with 18 data points. A bulge in the temperature profile (Fig. 8) is documented by shallow and medium deep sensors. Decreasing

Fig. 9. Model parameters for a singular mud flow.

temperatures above 1 mbsf and below 4 mbsf indicate a layer thickness of warm material of ∼4 m. FEMLAB® 3.0a is used for finite-difference method numerical calculations. To start modeling, we hypothesize that warm mud is produced at the center of HMMV, producing distinctive mud flows (Fig. 9). The thickness (a) of this individual mud layer is estimated from the temperature maximum at penetration H0310P07 (Fig. 5g) to be 4 m. The expulsion temperature T0 of this mud flow is suggested to be between 24 °C and 28 °C. The ambient seawater temperature above is around − 0.8 °C. The lower boundary condition is a basal heat flow of 170 mW/m2, deduced from the temperature gradient, found at site GC 2 (Table 3). Fig. 8 shows the best fitting result with parameters T0 = 25.5 °C and a time for temperature equalization of 2.9 days. Higher starting temperatures T0 result in inadequate temperature distributions. Simulations with flow thicknesses of 2.5–5.5 m, and initial temperatures of 24.5–26.5 °C all give ages between 2.5 and 3.5 days for this specific mud flow. Thus, we are confident that we found a mud flow, younger than 3 days with a core temperature of 25.8 °C. Taking the above-mentioned mud flow as a singular effect, it would take 3.5 months to achieve equilibrium (99%) with its surroundings. The modeling is most sensitive to the uppermost temperature values and least sensitive to the basal heat flow; thus, even position uncertainties for site GC 2, which constrains the deeper part of the section, are not critical to the modeling result.

Table 3 Parameters for mud flow age calculation Fig. 8. A composite temperature profile from 0 to 10 mbsf. Data between 0 and 0.55 mbsf are from the mini-lance, 1–4 mbsf are from Bremen heat probe and 5–10 mbsf are from gravity corer #7. Small dots are data points and larger dots represent temperatures, modeled for one mud flow of 4.5 m thickness, extruded 3 days before at a temperature of 26 °C.

Input

Output

T0 = 26–28 °C d = 3.5–5.5 m Q0 = 170 mW/m2 TTop = − 0.8 °C

t = 2.5–3.5 days

N. Kaul et al. / Marine Geology 229 (2006) 1–14

A second model hypothesis tries to elucidate the total volume of mud that has to be conveyed through the vertical conduit in order to feed a source of ∼26 °C warm mud in a cold environment. We assume a single circular feeder channel of width D (Fig. 9), rooted at 3000 mbsf. The geometry of this model is derived from seismic data, published by Hjelstuen et al. (1999). Two seismic lines (D2-82 and 7200-77) give evidence that the conduit of HMMVoriginates at 2.5–3 km depth, that is within the 3.1 km thick glacial unit, deposited since Pliocene times. Our model starts at this depth as the most probable depth of origin and a temperature T (3000 m) of 162 °C representing the lower boundary condition. This temperature corresponds to a mean regional heat flow of 65 mW/m 2 at a thermal conductivity of 1.2 W/m K. The value for the thermal conductivity was chosen from our measurements outside HMMV (i.e. H0310P01) and extrapolated to greater depth. The upper boundary condition of the feeder channel is given by the outflow temperature of 26 °C. Densities of mud within the conduit and surrounding material are 1950 and 2000 kg/m 3 , respectively. The specific thermal capacity of mud is taken as 1040 J/kg K. A number of simulations with varying diameters from 1 to 1200 m have been calculated in order to satisfy the thermal boundary conditions at the upper and lower end of the feeder channel (Table 4). Conduits narrower than 1 m are difficult to model, since the numerical calculation becomes instable. Our approach assumes a continuous upward flux of material. The result of the simulation is a velocity/diameter relation and hence a volume of material at the upper outflow with respect to channel width. We find that the velocity of upward migration changes by five orders of magnitude (Fig. 10a). In contrast to the velocity, the volume of mass outflow is in the same range of magnitude, varying from 10,000–30,000 m3/yr (Fig. 10b). The total amount of conveyed thermal energy, connected to the mass flow is 0.5–1 ⁎ 103 GJ/yr (Fig. 10c). The volume of mud appears to be remarkably indifferent to the width of the conduit and should be at

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Fig. 10. Results of modeling rates of mud expulsion with respect to conduit diameter. Lower: migration velocity vs. conduit diameter. Boundary conditions are temperatures at lower and upper end of conduit. Middle: mud volume expulsion does not exceed certain limits. Upper: thermal energy, transported with the expulsed mud corresponds to the amount of mud.

least 10,000 m3/yr to deliver mud of the observed temperature. The controlling factor for this is the heat flux through the shell of the conduit because the material loses 135 °C of its original temperature or 83% of its thermal energy during its ascent. To consider a conduit as wide as HMMV itself is not necessary in order to maintain a considerable amount of warm mud and a conduit smaller than 1 m in diameter would result in dramatically increasing velocities (above 12 km/yr). 5. Discussion

Table 4 Parameters for mud volume calculation T0 = −0.8 °C T3000 = 162 °C k = 1.2 W/m K ρ = 1950 kg/m3 ρ = 2000 kg/m3 H = 3000 m D = 1–1200 m

Temperature measurements in different depth ranges showed that none of the methods alone gives the entire picture of this thermally heterogeneous environment. Due to the limited position accuracy, estimated as ± 30 m for each of the instruments, it was possible only at one site to combine data from three methods into a longer gradient. However, this is the key site at the most active center of HMMV and is used for subsequent modeling.

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From the temperature measurements, we get three major insights: (1) the maximum temperature of material near the upper end of the conduit is about 26 °C, (2) very high vertical temperature gradients of 2000–3000 mK/m only occur near the surface and do not continue to greater depths and (3) there is a steep horizontal gradient in temperature at a distance of 300– 500 m from the eruptive center. These observations allow us to divide the measured temperature fields into two classes, one related to small scale surface effects and the other related to deep-seated and long-term effects. Looking at the small scale effects, based on modeling, we interpret that we were able to observe a mud flow less than 3 days old. This corresponds to seafloor observations with ROV “Victor 6000”, which show a homogeneous grey mud with absolutely no benthic bioactivity. Evaluating the high resolution bathymetry, this flow covers an area of 5000 m2 while the height is estimated as 3–4 m. The latter value is derived from the bulge-like temperature distribution and additionally from the bathymetric mapping. A calculated mud volume of 15,000–20,000 m2 is greater or equal to the total annual discharge of HMMV. Thus, we can estimate that a mud flow like the observed one occurs about once a year. The second aspect is the deeper structure of HMMV and the root of its supply channel down to 3000 mbsf, which was numerically modeled. Two seismic sections (D2-82 and 7200-77 of Hjelstuen et al., 1999) show downward bent reflectors for a horizontal extent of 1.0– 1.5 km underneath HMMV, exactly the size of the surfacial expression of HMMV. Hjelstuen et al. (1999) attributed this effect to a near surface velocity reduction. Horizontal resolution, expressed as the size of the first Fresnel zone, can be estimated as 500–700 m. A vertical channel of that size cannot be observed further downward. It is remarkable that the seismic expression of a vertical channel and the effective diameter of this channel differ greatly, as we have to consider that only small conduits are available for migration of mud and fluids within a broader chimney of mud breccia. A similar discussion is found in Krastel et al. (2003) for the Dvurechenskii Mud Volcano (Black Sea), where they can even trace seismic horizons through the apparent feeder channel. For low viscosity muds, effective pathways need not be wider than a few decimeters or meters, according to Poiseuille's and Stoke's laws and the observations compiled by Kopf and Behrman (2000). Looking for a classification of HMMV in comparison to other mud volcano phenomena, we find that HMMV is a mud pie rather than a volcano as described for the

Barbados accretionary prism (Henry et al., 1996). From analog modeling, it is known that there is a relationship between surface expression and width of the feeder (Lance et al., 1998). Mud pies indicate wide conduits, while mud domes result from narrow ones, using the same material. Wide conduits in this case mean diameters ranging from a few decimeters up to a few meters. Soft mud also means a high content of water within the transported material. As this material is a three-component substrate of mineral, water and gas, energy transport is predominantly coupled to the amount of water within the substrate because water is the most abundant component and specific heat capacity of water is four times that of the mineral components. The gaseous component is almost negligible with regard to energy transport. The effect of rapid sedimentation cannot be neglected at this site. The crustal heat flow related to 33–37 Ma old crust should be 80 mW/m2 according to the model of Parsons and Sclater (1977). It is convenient to consider only the glacial sedimentary unit because its sedimentation rate is more than 10 times that of the lower preglacial sequence. A mean rate of 1.35 mm/yr for 2.3 Ma results in a gradient reduction of 55% according to von Herzen and Uyeda (1963). We actually find values of around 52–65 mW/m2 outside the crater of HMMV. This indicates a bias to higher heat flow than the expected 40 mW/m2. Either an increased basal heat flow or a heat source within the terrigenous material of the glacial unit may be the cause of this bias. Thermal conductivity, increased by this magnitude, seems unrealistic. In comparison to this feature, the much smaller structure “Mound 11” off the coast of Costa Rica shows an only slightly disturbed temperature field even though the mound is active in terms of gas hydrate generation and CH4 degassing (Schmidt et al., 2005). 6. Summary and conclusion Close investigation of the thermal structure of HMMV reveals the dynamic behavior of this active source of mud, fluid and gas. Narrow spaced temperature measurements allowed an upper temperature limit for outflow of mud to be constrained. Together with the temperature estimate for the deep-seated source of material, this enables the volume of mud convection to be modeled. Having the upper and lower temperatures as known parameters of the conduit, the result is a set of velocity/diameter values. The heat content of the system is balanced by the amount of heat, dissipating through the mantle surface of the conduit. The surprising result is that the transported volume is relatively indifferent, at a

N. Kaul et al. / Marine Geology 229 (2006) 1–14

rate of 10,000–30,000 m3/yr for conduits larger than 1 m in diameter. The detection of a mud flow only 3 days old is constrained by three features (a) its core temperature of ∼ 26 °C, which can be anticipated to be the effluent temperature of the conduit, (b) the very steep gradient below the mud line, determined by the mini-lance and (c) the virtually unpopulated surface of the area, observed on ROV “Victor 6000” images. A process of successive mud flows cannot be the cause for the high temperatures of 20–23 °C below 10 mbsf because cooling of a singular mud layer would take only 3 months. Thus, mud flows at a frequency of one per year would cool to the ambient environment, before being superposed by a new one. Our conclusion is that only part of the conveyed heat reaches the surface while a substantial amount of energy and probably material is distributed at the bottom side of the mud pond. Cooling of this “underplated material” is achieved via conduction through the shell of the pond and by convection of fluids upwards. The upward migration of fluid and gas is proven by the evaluation of chemical and thermal gradients (de Beer et al., 2006). Furthermore, gas bubbles emerging from HMMV have been observed in significant volumes (Klages et al., 2004). HMMV has to be considered a three-phase system, where gas, water and sediment particles might have different behaviors during their uplift (de Beer et al., 2006). The thermal situation of HMMV as a whole must have been stable for at least thousands of years. The thermal aurora, indicated by the horizontal gradient, integrates the source strength with respect to time and temperature. The long-term temperature of HMMV should be between 15 and 30 °C. Higher temperatures would yield higher than observed temperatures outside the pond. Acknowledgements Many thanks go to Kapitän Domke of R/V Polarstern and his crew for supplying excellent seamanship during ARK XIX/3b. We are grateful to the Alfred Wegener Institute, namely Michael Klages, Michael Schlüter and Antje Boetius, for finance and support of this project. We thank Jeffrey Poort and an unknown reviewer, who helped to improve the paper. References Camerlenghi, A., Cita, M.B., Hieke, W., Ricchiuto, T., 1992. Geological evidence for mud diapirism on the Mediterranean Ridge accretionary complex. Earth and Planetary Science Letters 109, 493–504.

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